1. Field of the invention
This invention relates to linear scale type displacement measuring instruments, and more particularly to improvements in a linear scale type displacement measuring instrument including a hollow elongate case connected to one of two workpieces, between which a relative displacement is to be measured; a main scale received in and held by the elongate case and made of a material having a coefficient of thermal expansion different from that of the elongate case; elastic members arranged in the longitudinal direction of the main scale for connecting the main scale to the elongate case so as to hold the main scale in the elongate case; and an index scale connected to the other of the two workpieces to be measured and movable along the main scale; wherein the relative displacement between the two workpieces to be measured is measured from a relative movement between the main scale and the index scale, and suitable for use in a linear scale type displacement measuring instrument having a long main scale.
2. Description of the prior art
A linear scale type displacement measuring instrument is one type of length measuring apparatuses for measuring or adjusting the positional relationship between two objects.
This linear scale type displacement measuring instrument is of such an arrangement as shown in FIGS. 1 through 3, for example.
In the drawings, an elongate case 1 made of aluminium and produced by cold drawing for example has a hollow section of substantially square shape, is elongated in a direction perpendicular to the paper surface of FIG. 1, and has an opening 2 along one side surface in the longitudinal direction, over the substantially total length.
A detecting mechanism 3 as the movable member is brought into abutting contact with an end face of the elongate case 1 on the side of the opening 2 through a slide member 4, and is movable in the longitudinal direction of the elongate case 1.
An arm 5 extending from the opening 2 into the elongate case 1 is integrally formed on the undersurface of this detecting mechanism 3. A pair of magnets 6 are provided at positions close to the opening 2 on the outer side surface of the elongate case 1. A seal member 7 formed of a thin iron sheet is attracted by these magnets 6 in a manner to cover the opening 2, whereby dust and the like are precluded from entering the elongate case 1 through the opening 2.
At this time, the seal member 7 disposed at a portion, into which the arm 5 of the detecting mechanism 3 is inserted, is extended through a groove 8 having a convex cross section as viewed from sideways, opposite ends of which are opened in the undersurface of the detecting mechanism 3. The arm 5 strided over by this groove 8 is insertable into the elongate case 1.
Inserted into a groove 9 formed in the elongate case 1 in the longitudinal direction thereof is the lower end portion of a main scale 10 made of glass and formed at one side surface 10B (a graduated surface) thereof with a vertical fringe-shaped graduation 10A (Refer to FIG. 2). The main scale 10 is solidly secured to the elongate case 1 by means of a spacer 11A, an elastic rubber member 11B and a bonding agent 11C disposed in the longitudinal direction thereof.
The arm 5 of the detecting mechanism 3 is extended to a position close to the main scale 10, and a carriage 13 is movably secured to the forward end portion of the arm 5 through a connecting means 12. For example, this connecting means 12 includes a linear cantilever spring 12D integrally formed at the distal end thereof with a triangular loop portion 12A and secured at the proximal end thereof to the arm 5 through a washer 12B and a screw 12C; and a truncated cone 12E engageable with the aforesaid loop portion 12A.
The cantilever spring 12D is adapted to urge the carriage 13 towards the graduated surface 10B additionally functioning as a frist scanning reference surface of the main scale 10, as well as to urge the carriage 13 towards an end face 10C as a second scanning reference surface perpendicularly intersecting the graduated surface 10B of the main scale 10.
The carriage 13 includes a probe mounting member 13A formed into a substantially L shape from a plate; a light emitting element mounting member 13B having a large thickness, being screwed to a bent short side at one end of the probe mounting member 13A and opposed to a surface of the main scale 10, where the graduation 10A is not formed; and a light receiving element mounting member 13C having a large thickness, being screwed (not shown) to a bent long side at the other end of the probe mounting member 13A and opposed to the graduated surface 10B of the main scale 10.
An index scale 14 having a vertical fringe-shaped graduation, not shown, similar to that of the main scale 10 is solidly secured to the surface of the probe mounting member 13A of the carriage 13 in opposed relationship to the graduated surface 10B of the main scale 10.
Light emitting elements 15 as the light source and light receiving elements 16 are arranged such that the index scale 14 and the main scale 10 are interposed therebetween.
In this case, two light emitting elements 15 are solidly secured to the light emitting element mounting member 13B affixed to the L-shaped short side of the probe mounting member 13A, and also two light receiving elements 16 are solidly secured to the light receiving element mounting member 13C affixed to the L-shaped long side of the probe mounting member 13A.
Pluralities of sliders 17 and 18 made of a resin material having a low frictional coefficient are solidly secured to inner surfaces of the L-shape of the probe mounting member 13A, i.e., the surfaces opposed to the graduated surface 10B as the first scanning reference surface of the main scale 10 and the end face 10C as the second scanning reference surface perpendicularly intersecting the graduated surface 10B, respectively. These sliders 17 and 18 are adapted to abut against the graduated surface 10B of the main scale 10 and the end face 10C perpendicularly intersecting the graduated surface 10B through a biasing force of the cantilever spring 12D.
In the above-described arrangement, when one of either the elongate case 1 or the detecting mechanism 3 as the movable member, e.g., the detecting mechanism 3, is secured to a workpiece to be measured and the other, i.e., the elongate case 1, is solidly secured to a bed of a machine, i.e., a stationary side 19, and, if the workpiece to be measured is moved, then bright and dark fringe patterns are generated between the graduated surface 10A of the main scale 10 and the graduation of the index scale 14. Changes in brightness and darkness of these fringe patterns are read by the light emitting elements 15 and the light receiving eleements 16, whereby a movement value of the workpiece being measured is read, so that measurement can be carried out.
The linear scale type displacement measuring instrument as described above has as its feature that a displacement of the workpiece being measured can be digitally measured. In general, however, the elongate case 1 is made of aluminium and the main scale 10 received and held in this elongate case 1 is made of glass, thereby presenting such a disadvantage that a difference in value of thermal expansion occurs therebetween at the time of temperature change. In consequence, as shown in FIG. 1, heretofore, the main scale 10 positioned by one side surface of the groove 9 of the elongate case 1 and the spacer 11A provided at the bottom of the groove 9 has been pressed and held over the total longitudinal length of the main scale 10 through the agency of the elastic rubber member 11B solidly secured to one side surface of the aforesaid groove 9 by the bonding agent 11C. The above-described holding method can stably hold the main scale 10 as compared with the case of using a spring or the like, and moreover, the difference in value of thermal expansion between the main scale 10 and the elongate case 1 is adapted to be absorbed by the elastic deformation of the elastic rubber members 11B. However, recently, linear scale type displacement measuring instruments have been utilized in feedback control systems of the large-sized numerical control machines. The linear scale type displacement measuring instruments for the application described above are sometimes as long as four to six meters in total length. According to the conventional holding method as described above, the difference in value of thermal expansion between the main scale and the elongate case cannot be absorbed, whereby a thermal stress acting on the main scale 10 through the elastic rubber members 11B becomes excessively large at the time of temperature change, thus possibly causing damage to the main scale 10. The same is true of the case where the main scale 10 is bonded to and held by one side surface of the groove 9 of the elongate case 1 by an elastic bonding agent, e.g., a silicone rubber bonding agent applied to the main scale 10 over the total longitudinal length.